1. Field of the Invention
This invention relates to the catalytic conversion of hydrocarbons with a fluidized catalyst. More particularly, it relates to the catalytic cracking of a hydrocarbon stream injected into the catalytic cracking zone in the liquid phase. This invention especially relates to injecting a liquid hydrocarbon feed into a catalytic cracking zone together with dispersion steam in a manner which provides uniform distribution of hydrocarbon feed and steam.
2. Description of the Prior Art
Fluid catalytic cracking of petroleum fractions is a well-established refinery operation. The catalytic cracking apparatus usually comprises a reactor section where catalytic cracking occurs coupled with a regenerator section where coke deposited on spent catalyst is burned. The process operates essentially as follows. Fresh feed, which may be preheated, is mixed with catalyst and undergoes cracking within the reactor section. Products are removed from the reactor in the vapor phase and passed to a products recovery station comprising at least one main fractionator or distillation column for separation of the products into desired fractions. Spent catalyst, which has been coked by the cracking reaction, is continuously passed from the reactor to the regenerator by a spent catalyst transfer line. In the regenerator, the coke is burned by contact with an oxygen containing gas. Flue gas is passed from the regenerator, and regenerated catalyst is recirculated to the reactor via a standpipe where it is picked up by the fresh feed hydrocarbon charge stream. The catalyst itself is finely divided and simulates a fluid in various portions of the catalyst section, whence the name of the process. In a typical operation, heat generated in the regenerator is carried by the hot regenerated catalyst to the reactor to supply heat for the endothermic cracking reaction. Typical fluid catalyst cracking systems are disclosed in U.S. Pat. Nos. 3,206,393 of Pohlenz and 3,261,777 of Iscol, et al.
The fluid catalytic cracking process has been improved in efficiency over the years. In particular, the discovery of zeolite catalysts with their greater activity and reduced coke make, and improvements in design of the reactor section to emphasize dilute phase cracking, are cases in point. Systems for dilute phase cracking, also known in the art as riser cracking or transfer line cracking are typically disclosed in U.S. Pat. Nos. 3,261,776 of Baumann, et al., 3,448,037 of Bunn, et al. and 3,894,935 of Owen.
In the catalytic cracking of petroleum hydrocarbons, the fresh feed stream has been usually preheated before being injected into the reaction zone for contact with the cracking catalyst. In some instances, sufficient heat has been supplied to vaporize the hydrocarbons so that they were injected as a vapor. However, the energy required for complete vaporization often proved uneconomical. Alternately, injecting the hydrocarbons into the reaction zone in the liquid phase was employed. This often did not prove satisfactory since poor catalyst-oil mixing was obtained and excessive coking and attendant product loss were experienced. It was eventually found that injecting the hydrocarbon feed as a liquid in finely divided form, i.e., as atomized liquid droplets, prevented the undesirable effects of coking and product loss experienced when liquid feed was injected as a continuous individual stream into a catalytic cracking zone. The use of an atomized liquid feed has been usefully employed in both riser cracking and dense bed cracking units, although it was found that proper feed injection was less critical in a dense bed unit than in a transfer line reactor.
A variety of techniques has been employed in the art to provide a hydrocarbon feed in atomized form for use in the conversion zone. In U.S. Pat. No. 2,952,619 of Metrailer, et al., a heavy hydrocarbon feed is passed through the inner passage of a feed nozzle having a grooved circumferential surface which defines a threaded passage for the oil. Steam is passed through an annular passageway surrounding the oil passage. As the oil passes through its passageway a spinning component of motion is imparted to the oil forcing it outwards as a hollow cylindrical film. As the steam is emitted from the nozzle it shears the cylindrical film of oil shearing and atomizing it into relatively fine droplets. The nozzle of Metrailer, et al. is particularly useful in transfer line coking of residuum.
The oil feed system for a fluid cracking unit disclosed in U.S. Pat. No. 3,071,540 of McMahon, et al. employs two concentrically arranged nozzles. The nozzles are arranged to provide for concurrent flow of oil through the inner nozzle and steam through the outer nozzle. The inner nozzle terminates inside the outer nozzle so that as the oil passes from the exit of the inner nozzle it is contacted with high velocity steam and then the combination of steam and oil passes through the exit of the outer nozzle. The net result of this arrangement is to cause the steam to shear the hydrocarbon feed into relatively fine droplets. One or more of these nozzles is employed in the lower portion of a dense fluidized bed reactor with a single nozzle arranged in a riser or transfer line reactor.
A swirling or spiral motion similar to that imparted to the oil in the nozzle of Metrailer, et al. is employed in the oil-steam nozzle disclosed in U.S. Pat. Nos. 3,152,065 of Sharp, et al. and 3,654,140 of Griffel, et al. In one of the embodiments of Griffel, et al., the liquid hydrocarbon feed passes through the tube of the nozzle which is provided with a spiral or helix to cause the hydrocarbon to exit from the nozzle in the form of a hollow conical sheet. Surrounding the oil feed tube is a larger diameter tube concentrically disposed about the inner tube. Steam passes through and emmanates from the annular passageway contacting the expanding cone of oil, rapidly breaking it into discrete and small droplets. The nozzle of Sharp, et al. also consists of two concentrically disposed tubes but here the oil passes through the annular passageway and the steam through the inner tube. The annular passageway is provided with a helical assembly to impart a spiral motion to the hydrocarbon feed. The outer tube extends past the end of the inner tube and is provided at its downstream end with an orifice plate only slightly larger in diameter than the internal diameter of the inner tube. The centrifugal swirling motion imparted to the oil causes it to pass through the restriction orifice in the form of an annular cone-shaped wall of liquid. The steam passing inside this liquid annulus causes the liquid to break up into uniform liquid hydrocarbon droplets. In optional embodiments, a plurality of nozzles may be installed in a vertical riser cracking line to discharge the feed stock vertically or at an angle to the direction of flow.
It has been found that nozzles employing a helical or spiral device to impart a swirling motion to the oil can encounter plugging problems, particularly with heavy feedstocks, and require higher inlet pressures than other types of injection means.
It has also been found that although multiple nozzles have been employed to provide uniform distribution of feedstock across the cross section of the reaction zone thereby achieving efficient contacting of oil and catalyst, it has not been possible heretofore to insure that all nozzles receive equal quantities of steam and oil when each is supplied from a common source so that the desired uniform distribution is often not obtained.
It is an object of this invention to provide a process for supplying uniform quantities of steam and oil to each discharge port of a multiple port oil-steam nozzle.
It is another object of this invention to provide a multiple port oil-steam nozzle which will supply a uniform distribution of oil feedstock across the cross-section of a reactor.
It is a further object of this invention to provide a process for supplying a continuous stream of atomized liquid hydrocarbon to a catalytic reaction zone.
The achievement of these and other objects will be apparent from the following description of the subject invention.